This invention relates generally to the measurement of elliptical elements on a medium and particularly to the measurement of the dots in a kinescope faceplate panel, or shadow mask.
During the production of picture tubes for color television receivers, a black matrix material is applied to the inside surface of the faceplate panels. The black matrix coating includes a large number of transparent dots which are arranged in a particular pattern. The transparent dots are coated with slurries of photosensitive materials including phosphors which emit the three primary colors of light when impacted by electrons. The phosphors are alternately applied in a repetitive sequence such as red, green, blue, to the transparent dots. Prior to the application of the phosphors, it is desirable to measure the widths of the transparent dots, and the opaque matrix spaces between the dots, to verify, that they are within acceptable dimensional tolerances to avoid the expensive application of phosphors to improperly matrixed panels.
In order to measure the transparent dots, and the opaque spaces between the dots, the faceplate panel is placed between a stationary light source and a detector enabling light to pass through the dots to the detector. Portions of the panel are scanned with light and variations in the light transmission capabilities of the opaque spaces and the transparent dots are detected and provided to a measuring system.
Typically, the detector is a photosensitive solid state device, such as a linear photodiode array or a CCD. The pixels of the array are charged to various levels in accordance with the amount of light impacting the individual pixels. A camera is used to focus the matrix dot pattern onto the array. The camera lens focuses and magnifies the image so that each dot spans a number of detector pixels. Scanning is affected by rotating a galvo to change the portion of the dot pattern which is focused onto the array. The pixel charge levels are periodically clocked out to effectively scan a large number of lines across the dot pattern.
Ordinarily the dot measuring data are gathered from selected portions of the panel rather than from the entire panel. Accordingly, measurements typically are made at the four corners and center of the panel. The dot patterns on the faceplate screens are made by photoexposing the photosensitive materials through an apertured element, called a shadow mask. Accordingly, the light rays which pass through the apertures in the shadow mask to expose the photosensitive materials, strike the photosensitive material at different angles. This causes the dot elements which are in the proximity of the corners of the panel to be elliptically shaped rather than circular, as is desired. For this reason, for high definition television tubes, the dimensions of the dot elements must be measured along both the major and minor axes of the elements. Accordingly, in order to ensure that all the measurements can be made in the time allotted for each panel, typically 15 seconds, it is necessary to simultaneously scan all the selected areas along both the major and minor axes of the elliptical elements and to gather the data separately for each of the dimensions and for each of the measuring areas. For these reasons, there is a need for a system for simultaneously measuring elliptical patterns along both the major and minor dimensions of the elliptical patterns. The present invention fulfills this need.